1. Field of the Invention
The present invention generally relates to a composite filter medium having two or more filtration zones or layers of different particle-retention capability (xe2x80x9cPRCxe2x80x9d) with respect to the fluid suspension which is filtered under ambient filtration conditions (xe2x80x9ccomposite filter mediumxe2x80x9d) wherein the zones or layers are positioned with respect to one another such that the contaminant-holding capacity (xe2x80x9cCHCxe2x80x9d) per unit area of the composite filter medium is greater than the CHC per unit area of the upstream filtration zone or layer of the composite filter medium when such is extrapolated to the depth of the composite filter medium. More particularly, an embodiment of the present invention relates to a cell-type filter unit employing such composite filter medium. More particularly, an embodiment of the present invention relates to a cell-type filter unit having upper and lower composite filter medium separated by a separator layer wherein the zones or layers of each composite filter medium are positioned with respect to one another such that the CHC per unit area of the composite filter medium is greater than the CHC per unit area of the upstream filtration zone or layer of the composite filter medium when such is extrapolated to the depth of the composite filter medium. More specifically, an embodiment of the present invention relates to a cell-type filter unit having an upper composite filter medium and a lower composite filter medium separated by a non-filtering separator layer, wherein each of the composite filter medium is comprised of two or more zones or layers of filter material of the same or different composition and/or fabrication, each layer being positioned with respect to one another such that the more distal the zone or layer from the separator layer the lesser the PRC with respect to the fluid suspension which is to be filtered under attendant filtration conditions. And yet another embodiment of the present invention relates to a lenticular filter unit having an upper composite filter medium and a lower composite filter medium, separated by a non-filtering separator layer, wherein each composite filter medium is fashioned to have a graded PRC in the direction of flow such that, as positioned on the non-filtering separator layer, the lenticular filter unit is capable of more efficiently retaining smaller and smaller particles as the fluid moves from the surface of the filter medium towards the non-filtering separator layer.
2. Background of the Related Art
Cell-type filter units are well known in the art, and comprise two overlying similarly-shaped filter media separated from one another along the majority of their opposing surface areas by a non-filtering separator element, and affixed to one another along their perimeter edges. Conventionally, the filter media and the separator element each have a central void of about the same shape and dimension so as to form a uniform through bore in the filter unit when each void is aligned.
The separator element is conventionally composed of a material distinct from the composition of the media which abuts it, and generally has openings therein of such size that the separator is substantially non-filtering with respect to the material to be filtered given its position within the cell-type filter unit. In addition to separating the two filter media, and supporting the media under differential pressure, the separator element is generally fashioned to have a plurality of conduits formed therein, such conduits communicating with the central void of the separator and the through bore of the filter unit to allow flow to get from the outer-diameter or edge of the cell-type filter unit to a stacked common bore. Separators are conventionally fashioned from polymeric materials, in particular plastics, although they can also be fashioned from other materials, such as, for example, metals, ceramics and other material known in the art to be capable of separating the two layers effectively in a particular filter application environment.
A separator element may be manufactured to include upper and lower ribs of varying thickness to maintain the media in a disk-shape. Lenticular cell-type filter units, comprising two disk-shaped filter media separated by a closed-curve non-filtering separator element, are particularly common place in the art. Separators used in lenticular filters generally have a plurality of ribs extending radially outward from a central aperture in a spoke-like fashion. An example of a lenticular cell-type filter unit is found in U.S. Pat. No. 4,783,262 to Ostreicher et al., the disclosure of which is herein incorporated by reference.
Generally the outer circumference of the two media discs of a lenticular filter unit are held together by an insert molding process which encapsulates the circumferences in plastic. U.S. Pat. No. 4,347,208 to Southhall, the disclosure of which is herein incorporated by reference, discloses a method of making a filtration cell having a sealed periphery which includes the steps of placing two media discs, and interposed separator, into a mold and injecting a thermoplastic polymer into the mold to form a seal around the two media discs. The Southhall patent discloses polypropylene, polyethylene, nylon, and polysulfone as the preferred thermoplastic polymers for molding the edge seal.
Cell-type filter units use a variety of materials for filtering fluids, including, without limitation, glass fibers, diatomaceous earth, perlite, cellulose, and binder resins. The filter media is typically produced by a wet laid papermaking operation. Media thickness generally ranges between about 0.130-0.218 inches depending on the material formulation. By filter medium it is meant a porous article or mass having a porosity, or carrying/producing a charge, or incorporating matter which binds matter in the suspension, such that it will separate out matter in suspension in the fluid, gas or liquid, which is to be filtered.
Cell-type filter units generally have a through bore and are generally employed in conventional practice by stacking one on another in seriatim to form a common bore, such common bore communicating with one or more separator conduits. The stacked cell-type filter unit assembly, or cell-type filter cartridge, is then enclosed in a housing having an inlet port and an outlet port, the common bore typically being positioned in the housing so as to communicate with the outlet port. Not infrequently, fluid is supplied to the housing at high temperature and/or high pressure. The fluid enters the gaps between the adjacent filter units and then passes through the filter media covering the separator. As the fluid passes through the filter media, undesirable materials such as aggregates and particulates are removed from the fluid. The filtered fluid then flows along the conduits of the separator to the common bore and exits the housing via the outlet tube.
A significant advantage of stacked cell-type filter cartridges is that the surface area of the filter material is quite large when compared to the total volume displaced by the stacked cell-type filter cartridge. This large surface area permits larger volumes of fluid to be filtered, as compared to cartridges displacing a similar volume but which have a lower surface area, over the same period of time. Conventional stacked cell type filter cartridges are useful in a variety of applications, including the filtration of fluids such as beverages, dielectric oils, chemicals, etc. Cell-type filter cartridges find use as both primary filters and pre-filters.
When used as pre-filters, stacked cell-type filter cartridges may be located upstream from another stacked cell-type filter cartridge, or from a filter cartridge of dissimilar construction, e.g. a pleated membrane filter. Owing to their large available surface areas cell-type filter cartridges are frequently used to remove particulates from a fluid stream prior to microfiltration by a membrane filter. The pre-filter is designed to remove particulates which would otherwise plug the membrane, thereby reducing both the filtration flow rate (or at constant flow, increasing the pressure differential through the membrane filter) and reducing the life of the membrane filter. While such dual filtration systems result in a highly purified effluent, the cost involved in maintaining both the pre-filter and qualifying filters is relatively high. Additional operational costs are incurred in using multiple filters in that additional housings must be purchased and installed to incorporate each succeeding filter. Further, there is a downtime cost with respect to the replacement of either filter, one filter not infrequently being optimally replaced at a different time than the other filter.
While multi-layer cell-type filter units are known in the art, additional layers serve purposes other than to increase CHC. For example, Cuno 05UW Zeta-Plus(copyright) is constructed of two identical cellulose and glass fiber layers (having the same pore size distribution and charge potential, as well as the same CHC per unit area and PRC) having a water absorbent layer of different materials located there-between. The water absorbent layer is interposed to remove water from an oil filtrate and does not act as a particulate filter medium. The cellulose layers act both as particle retention filters and also as support for the relatively weak water absorbent layer as it swells. A filter of similar construction is also produced commercially by Alsop(copyright). Zeta-Plus(copyright) filters are also available having a layer of spunbond polypropylene or polyester non-woven placed between the separator and the cellulosic filter media. The interposed layer does not act as a filter medium, but rather is used to support the filter media, in particular under differential pressure. Zeta-Plus(copyright) filters having a layer of spunbond or netting placed on the outer surface of the filter media are also known. Such outer layer is used to provide support in a reverse flow/pressure condition and helps insure that fluid flow is not obstructed between cells if the media faces of two adjacent cells are in contact. Flowtech(copyright) also produces a similar commercial product. In neither case does the outer layer act as a filter medium.
A multi-layered construction is also found in the Roki Techno ABSO-AB(copyright) Series lenticular filters. In this product two cellulosic filter media layers are disposed on each side of the separator. One thin layer of melt-blown material, of about half the thickness of the overlying cellulosic filter media, is located under the two-layer cellulosic filter media, in contact with the separatorxe2x80x94that is the melt-blown material is located between the separator and inner cellulose media layer. The melt-blown material layer is used to reduce medium migration from the cellulosic filter media to the separator. Such melt-blown material layer does not increase particle retention over the cellulosic filter media. The melt-blown material layer, as measured by a Coulter Porometer, has a 12xc2xd micron mean flow pore size versus 2-4 micron mean flow pore size for the cellulose filter media.
Japanese Utility Model 5-2709 also discloses a multi-layer lenticular cell-type filter unit but does not describe the particle retention properties of the layers. No teaching or suggestion is made to incorporate filter medium having two or more layers and/or zones of different PRC, with respect to the fluid suspension, which is filtered, under attendant ambient filtration conditions.
There is, therefore, a need for a more economical filtration system that results in decreased down time due to filter replacement and to provide for highly purified effluent without the need to resort to a dual filter filtration system. Further, it is desirable that the useful life of any qualifying filter used in a process be extended.
Disclosed is a cell-type filter unit having upper and lower filter media composed of two or more filtration zones or layers of different particle-retention capability (xe2x80x9cPRCxe2x80x9d) with respect to the fluid suspension which is filtered under ambient filtration conditions (xe2x80x9ccomposite filter mediumxe2x80x9d) wherein the zones or layers are positioned with respect to one another such that the contaminant-holding capacity (xe2x80x9cCHCxe2x80x9d) per unit area of the composite filter medium is greater than the CHC per unit area of the filtration zone having the highest PRC on a basis weight (gm/sq-cm) comparison. More particularly, an embodiment of the present invention relates to a cell-type filter unit having an upper and lower composite filter medium separated by a separator layer wherein the zones or layers of each composite filter medium is positioned with respect to one another such that the CHC per unit area of the composite filter medium is greater than the CHC per unit area of the filtration zone or layer of the composite filter medium which has the greatest PRC on a basis weight (gm/sq-cm) comparison.
Particle retention by a filter medium may result, for example, from mechanical (e.g., pore size), chemical (e.g., covalent, hydrophilic bonding) or electro-kinetic interactions (e.g. anionic, cationic binding) between the suspended material which is to be removed and the filter medium.
Particle-retention capability (xe2x80x9cPRCxe2x80x9d) is a measure of the competence of filter medium to retain a diverse size range of particles. When two filter media are indicated to have xe2x80x9cdifferent PRCsxe2x80x9d it is meant that there is a measurable difference in either the relative-PRC or standardized-PRC. By increased xe2x80x9crelative-PRCxe2x80x9d of a first filter medium over a second filter medium, it is meant, that given the suspension being filtered, at ambient filtration conditions (pressure, temperature etc.), that the first filter medium is capable of removing particles of smaller size, and/or removing a given particle size more efficiently, than the second filter medium before a significant pressure drop across the medium occurs. As the PRC of a filter medium zone or layer may be affected by numerous parameters depending on the extreme of conditions and the method(s) of particle retention, for example, the pH of the fluid being filtered, the charge on the particles being filtered, the charge on the filter medium, the fluid pressure at which the fluid is filtered, the temperature of the filtered suspension, and the characteristics of the fluid in which the particles are suspended (e.g., bonding affinity between the fluid and the particles), a standardized-PRC measurement has been developed to characterize the ability of filter media to retain a diverse size range of particles with respect to commonly filtered suspensions under commonly encountered filtration conditions. By xe2x80x9cstandardized-PRCxe2x80x9d it is meant the smallest particle size that one basis weight (1 gm/1 sq-cm) of substantially uniformly-fabricated filter medium is able to consistently retain, before a significant pressure drop across the medium occurs, when the filter medium is challenged with 0.2 um-1.0 um diameter spherically-shaped mono-dispersed latex beads (of anionic charge if the filter medium is predominantly positively charge, of cationic charge if the filter medium is predominantly negatively charged, and of neutral charged if the filter medium is predominantly neutrally charged, using serial testing at 0.1 um diameter intervals) suspended in a solution of doubly-distilled water (adjusted to pH 4.0 if the latex beads are anionically-charged, to pH 8.0 if the latex beads are cationically-charged, and to pH 7.0 if the latex beads are neutrally-charged) when such latex beads are suspended at a concentration of 1 mg/deciliter and when such suspension is filtered at STP. When a first filter medium is said to have a xe2x80x9cdifferentxe2x80x9d standardized-PRC or relative-PRC than a second filter medium, it is meant that the relevant measurement differs by more than about 10%, and more preferably by more than about 25%, and yet more preferably more than about 50%. xe2x80x9cContaminant holding capacityxe2x80x9d is a measure of the ability of a unit area of filter medium to retain contaminants. When two medium are indicated to have xe2x80x9cdifferent CHCsxe2x80x9d it is meant that there is a measurable difference in either the relative-CHC or standardized-CHC. By increased xe2x80x9crelative-CHCxe2x80x9d of a first filter medium over a second filter medium, it is meant, that for the suspension being filtered, at ambient filtration conditions (pressure, temperature etc.,), that for given a unit area of projected filter medium (that is, projected along its thickness), the first filter medium is capable of retaining more of the particles suspended in the filtered suspension per unit area as opposed to the second filter medium, that is, before a substantial pressure drop across either filter media occurs. As with PRC, due to the number of variables that may affect CHC of a filter medium, including for example, the pH of the fluid being filtered, the charge on the particles being filtered, the charge on the filter medium, the fluid pressure at which the fluid is filtered, the temperature of filtered suspension, and the characteristics of the fluid in which the particles are suspended (e.g., bonding affinity between the fluid and the particles), a standardized-CHC per unit area measurement has been established to characterize the capacity of a projected unit area of most filter media to retain contaminants given exposure to most commonly filtered suspensions and under common filtration conditions. By xe2x80x9cstandardized-CHCxe2x80x9d it is meant the capacity (weight/weight) of a projected area of filter medium, before a significant pressure drop across the medium occurs, to retain a uniformly distributed diverse size-range of spherically-shaped monodispersed latex beads (of anionic charge if the filter medium is predominantly positively charged, of cationic charge if the filter medium is predominantly negatively charged, and of neutral charge if the filter medium is predominantly neutrally charged) having diameters of 0.2 um-1.0 um, at 0.1 um diameter intervals, when such beads are suspended in a solution of doubly-distilled water (adjusted to pH 4.0 if the latex beads are anionically-charged, to pH 8.0 if the latex beads are cationically-charged, and to pH 7.0 if the latex beads are neutrally-charged) when such latex beads are at a concentration of 1 mg/deciliter and when such suspension is filtered at STP. When a first filter medium is said to have a xe2x80x9cdifferentxe2x80x9d standardized-CHC or relative-CHC per unit area than a second filter medium, it is meant that the relevant measurement differs by more than about 10%, and more preferably by more than about 25%, and yet more preferably by more than about 50%, than the second filter medium.
An embodiment of the present invention includes a cell-type filter unit comprising: an upper filter medium element; a lower filter medium element, a non-filtering separator element disposed between the upper filter medium element and the lower filter medium element, and a sealing edge operatively connecting said elements along their edges; wherein the lower and upper filter medium are each comprised of at least two zones of filter material, each zone having different PRC, such that at least one zone of each medium is disposed proximal to the separator element and at least one zone of each medium is disposed distal to the separator element. The zones may be integral with one another or separate layers operatively connected to one another.
In a particularly preferred embodiment of the present invention, the upper filter medium elements and lower filter medium elements on each side of the separator element of the cell-type filter unit comprise, or consist of, 30%-50% cellulose, (e.g., Weyerhaeuser Kraft Kamloops(trademark)), and balance conventional filter aids (50%-70%), such as diatomaceous earth (e.g., Celite 507(trademark), Standard SuperCel(trademark)), and perlite (e.g., Harborlite(trademark)), and are generally of the same composition. The upper filter medium elements on each side of the separator element are fabricated in such a manner (as would be known by those of ordinary skill in the artxe2x80x94including changing the grade of the filter aid used, or the method or degree of refining/fibrillation of the pulp) such that the overall average pore in the media is substantially more open than those pores found in the lower filter medium elements. Differences between the average pore size between the upper filter medium element and lower filter medium element on each side of the separator element should eventuate in a difference in airflow pressure across the filter medium element of more than about 10%, more preferably more than about 25%, and yet more preferably more than about 50%. Preferably the dimensions of the upper filter medium element and lower filter medium element on each side of the separator are substantially the same.
Another embodiment of the present invention includes a cell-type filter unit comprising: an upper filter medium element having top, bottom and edge surfaces; a lower filter medium element having top, bottom and edge surfaces; a non-filtering separator element disposed between said bottom surface of said upper filter medium element and said top surface of said lower filter medium element in such a manner to be anterior to said lower filter medium element and posterior to said upper filter medium, and a sealing edge operatively connecting said lower and upper filter medium elements along their edges, wherein said lower and upper filter medium have a graded PRC from said top surface to said bottom surface of said filter media such that when a suspension containing a diverse particle-size distribution flows from said top surface to said bottom surface more small particles are retained as the depth from the top surface increases.
And yet another aspect of the present invention includes a cell-type filter unit comprising: an upper filter medium element having top, bottom and edge surfaces; a lower filter medium element having top, bottom and edge surfaces; a non-filtering separator element disposed between said bottom surface of said upper filter medium element and said top surface of said lower filter medium element in such a manner to be anterior to said lower filter medium element and posterior to said upper filter medium, and a sealing edge operatively connecting said lower and upper filter medium along their edges, wherein said lower and upper filter medium have a graded pore-size from said top surface to said bottom surface of said filter media such that a larger number of relatively larger pore sizes are found preferentially toward the top surface, whereas a larger number of relatively smaller size pore sizes are found toward said bottom surface of the filter media, and pore size varies as a function of depth into the filter medium.
A further embodiment disclosed is a cell-type filter cartridge comprising: a plurality of cell-type filter units, each cell-type filter units having an upper filter medium element surrounding a central void; a lower filter medium element surrounding a central void, a non-filtering separator element surrounding a central void disposed between the upper filter medium element and the lower filter medium element, and a sealing edge operatively connecting lower and upper filter medium elements along their edges, mounted generally parallel to and spaced from one another such that the a central continuous bore is formed there-between, wherein the filter media of the cell-type filter units are each comprised of at least two zones of filter material each layer having a different PRC. The zones may be integral with one another or separate layers operatively connected to one another.
Still another aspect of the present invention includes multi-layer filtration media prepared by a process comprising the steps of: providing a first set of filter media, each filter medium having about the same dimension, shape and PRC; providing a second set of filter media, each filter medium having about the same dimension and shape as, and having a PRC different than that of said first set of filter media; providing a separator element of about the same shape and dimension as said filter medium of said first and second set of filter media, said separator element significantly lacking filtering capability; operatively assembling the first set of filter media, the second set of filter media and the separator element to form a composite structure; and operatively joining the filter media of said composite structure along the edges of the filter media to seal the outer edge thereof.